Rules regarding buried earth cables

Nothing I can see in the regs differentiates between an earth cable and T&E when it comes to the rules regarding buried cables.

But what I can't work out is why it's OK to run a T&E in EARTHED conduit outside of safe zones, but NOT an earth cable, since from the perspective of a nail/screw penetrating into the wall and catching the cable or conduit - there is no difference.


  • Either I have misunderstood or this is an odd question.

    if you grab hold of the live nail trust me, you will feel the difference ;-)

    The idea of the earthed conduit is that the nail is already earthed to the conduit or cable armour, before it strikes the live core, so firstly the peak exposed voltage is reduced (being the voltage division of the resistances of  the live path to and the earth path from the point of fault and secondly, the exposed voltage is not there for long as the high current operates some sort of ADS on the supply side.

    The minimum sizes of lone earth wires are larger than those in cables like T and E to reduce the risk of being cut through by such abuse.

    Mike

  • There is nothing to stop you running a cpc in the conduit, but there may be the option of using the conduit itself as the cpc.

    Or perhaps by "earth cable" you mean the earthing conductor or a bonding conductor, which may need to be chased into a wall? Taken as a whole, I would say that a strict interpretation of 522.6.201 to 204 includes them, but common sense does not.

  • Generally it’s fairly obvious if someone cuts through a twin and earth cable because there’s a big flash and a bang and/or things stop working, but with an earth conductor damage may not be immediately evident, it is also less likely to get repaired.

    So a buried earth conductor really needs more mechanical protection, not less.

  • I suspect the OP is alluding to an earth cable by its self buried in a wall outside of the "safe Zones", and if that's OK or not.

    He's possibly asking why an earthed conduit may be buried outside of the "safe Zone" (with twin and earth in it, i assume) but an earth by its self can't be buried outside of the safe zone similar to earthed metal conduit. 

    My reply to this assumption on my part, would be, the earth cable (bonding cable perhaps?) buried in a wall outside of a safe zone, while electrically safe, is very poor practice and, in future, could more easily be cut by a wall chaser for example or damaged some other way and possibly be partially or completely disconnected in future works. Just because the original installer was too lazy to run it neatly and in the expected zones, where an electrician might expect cables to be.

    I'd not be happy with an earth run outside of the expected zone of where one might expect to find cables. 

    The metal conduit would be very difficult to cut without knowing you've cut it but a copper cable you can easily cut and never know it with a wall chaser. 

    Just poor practice rather than against the regs I suspect. 

  • I take it we're talking about cables concealed in a wall, rather than buried in the ground.

    rather than against the regs I suspect. 

    I guess we're talking the subtleties of 522.6.204 (i) - where a thick G/Y bonding or Earthing conductor would in principle satisfy the requirement of having an earthed metallic covering (if surrounding a non-existent core), but 6491X G/Y singles comply with BS 6004 rather than any of the cable standards stated.

    My first guess is that 522.6.204 was originally written only with cables containing live conductors in mind (it's also tied up with a requirement for 30mA RCD protection, which also doesn't make a lot of sense in some particular circumstances).

       - Andy.

  • "Just because the original installer was too lazy to run it neatly and in the expected zones, where an electrician might expect cables to be."

    This is not always possible.

    As it happens, in the instance which brought this up, I was able to drag the new earth cable through the path taken by the original earth cable (which needed renewing), but it was by no means a given for this to be possible.
    With a power cable, you can always get around  safe zone requirements by installing a socket to open up new allowed pathways. You don't have this option with an earth cable. Had it not been possible to reuse the same route, in this occasion the new earth would have required to be surface mounted which is going to be a hard sell to many home owners. Particularly when had it been a T&E multiple options for routing outside safe zones exist.

    Which is why I believe standalone earth cables are an oversight in the regs.